1. Field of the Invention
The present invention relates to a biodegradable plastic composition, a molded article thereof and a method for controlling biodegradation rate; in more detail, the present invention relates to a biodegradable plastic composition enabling to adjust biodegradation rate, improve hydrolysis resistance and heat resistance, and retain transparency by compounding a carbodiimide compound and an antioxidant to the biodegradable plastic, applications thereof such as a biodegradable molded article and a method for controlling biodegradation rate of the biodegradable plastic.
2. Description of the Prior Art
Recently, research and development of biodegradable plastics have been promoted on the background of increasing attention to the need and significance of environmental preservation, while problems such as environmental contamination caused by plastic wastes have been attracting attention.
The biodegradable plastics are roughly classified into aliphatic polyesters having polyester groups in a molecular backbone thereof (including those produced by microorganisms), natural macromolecule type polymers such as cellulose, polysaccharides produced by microorganisms, polyvinyl alcohol (PVA) and polyethers such as polyethylene glycol (PEG).
Among them, aliphatic polyesters have not been utilized as the biodegradable plastic because they have, in general, low melting points and poor heat stabilities in their production stages, and a sufficiently large molecular weight has not been obtained to provide physical properties suitable for practical molded articles. However, with technological developments to solve these problems, a high molecular weight of aliphatic polyester has come out, and started to be used in such fields as materials for agriculture, forestry and fishery (mulch film, plantation pot, fishing line, fishing net, and the like), materials for civil engineering work (water holding sheet, net for plants, sandbag, and the like) and packaging and containers field (those difficult to be recycled due to adhered soil, foods, and the like).
The biodegradable plastics including the above-described aliphatic type polyesters, however, should not only have the same level of functions as the conventional plastics (in characteristics such as strength, water resistance, formability and heat resistance) in use, but also be rapidly degradable by microorganisms generally present in the nature in disposal thereof.
Based on these situations, several methods for controlling the degradation rate of the biodegradable plastic have been proposed, for example, a method aiming at shortening of the degradation period by an addition of hydrolase (JA-A-4-168149), and another method aiming, contrarily, at an extension of the degradation period by reducing unreacted monomers and impurities in a polymer and low molecular weight compounds such as linear or cyclic oligomers (JA-A-9-12688), are known.
Notwithstanding these proposals, however, the biodegradable plastic consisting of the aliphatic polyester according to the conventional arts has currently such problems as a hydrolysis reaction progressing in the aliphatic polyester induced by exposure to moisture in an ambient atmosphere or by being heated during a production process of pellets thereof, which are raw materials of plastic products, or in a production process of products from said pellets, leading to lowering or dispersion of initial properties of molded articles as well as an unstable biodegradability of products thereof, and thus the biodegradability, that is, an adjustment of the biodegradation rate, is not yet sufficient.
The present inventors proposed, in JA-A-11-80522, a biodegradable plastic composition with a controlled biodegradation rate by compounding a carbodiimide compound into the biodegradable plastic. Although this proposal improved the hydrolysis resistance or a function for adjusting biodegradability, the improvement in heat resistance was not enough. The proposal also had another problem of losing transparency of product, in particular, in a poly(lactic acid) type biodegradable plastic product with an increased compounding amount of the carbodiimide compound to enhance the hydrolysis resistance.
JA-A-2001-525473 (WO99/29768) also proposes a method aiming at an extension of the degradation period by compounding an aromatic carbodiimide as a hydrolysis stabilizer to the biodegradable polymer; however, the method had a problem of losing transparency of biodegradable plastic products irrespective of the addition amount.